Polytetrafluoroethylene (PTFE) is well known for its outstanding chemical, physical and electrical properties. The unique combination of properties of PTFE has made it a particularly valuable plastic for a variety of applications including electrical wire insulations, bearing pads, gasketing materials and pipe linings. However, a disadvantage of PTFE is its extremely high melt viscosity which makes melt fabrication of the polymer impossible. A solution to this problem was found by copolymerizing TFE with certain perfluorinated monomers. In particular, copolymerizing tetrafluoroethylene (TFE) with hexafluoropropylene (HFP) has been found to provide tough, stable melt extrudable TFE/HFP copolymers (see e.g., U.S. Pat. No. 2,946,763). Copolymerizing TFE with perfluoro(alkyl vinyl ethers), e.g., perfluoro(propyl vinyl ether), (PPVE), has also been found to provide tough, stable melt extrudable perfluorocarbon polymers such as TFE/PPVE copolymers (see e.g., U.S. Pat. No. 3,132,123).
A TFE/HFP copolymer, which is used commercially under the name Teflon FEP resin, is capable of being fabricated at practical rates into thin, continuous shaped articles such as film, tubing and the like by conventional thermoplastic processing techniques. Although these TFE/HFP copolymers have outstanding properties and are used in a variety of applications, they have some disadvantages relative to PTFE homopolymers. For example, in order to lower the melt viscosity sufficiently to permit melt fabrication of the TFE/HFP copolymer and still obtain good toughness, more than 15 weight percent of HFP is usually incorporated in the copolymer. This has the effect of lowering the melting point of the copolymer by at least about 60.degree. C. relative to that of PTFE. Further, the large amount of HFP present has an adverse effect on the high temperature mechanical properties, e.g., it lowers tensile strength, of the copolymer. As a result, the upper use temperature for usual applications of these TFE/HFP copolymers is more than 60.degree. C. lower than that of PTFE. Also, these TFE/HFP copolymers possess a lower MIT flex life relative to PTFE homopolymer. A need existed for development of a fluorocarbon resin having the low melt viscosity of TFE/HFP copolymers but without the attendant lowering of tensile strength at high temperatures and lowering of the flex life.
The need referred to above was partially met by copolymers of tetrafluoroethylene and certain perfluoro-(alkyl vinyl ethers). These copolymers (referred to herein as TFE/PAVE copolymers) have good high temperature mechanical properties and toughness, and have melt viscosities low enough to permit fabrication by conventional thermoplastic processing techniques. However, good properties of these thermoplastic copolymers are achieved only with more than 3 weight percent of the vinyl ether present. The high cost of these vinyl ether monomers precluded their use in many applications and has led to continued interest in developing improved melt fabricable tetrafluoroethylene copolymers with little or no cost penalty.
Moreover, an important use of the TFE/HFP copolymers is in making heat-shrinkable tubing and the TFE/PAVE copolymers were less satisfactory for this use than the TFE/HFP copolymers because of their higher instantaneous "snap-back" tendency when stretching forces are removed.
Thus, a need existed for a fluorocarbon resin having the low melt viscosity with good tensile strength at high temperatures and good flex life, and having low "snap-back" tendencies.